An antibody-drug conjugate (ADC) technology is a novel target-oriented technology for causing apoptosis of cancer cells by releasing a toxic material in the cells after binding a toxin to an antibody bound to an antigen. Since the ADC technology may allow a drug to be accurately delivered to target cancer cells and released only under a specific condition while minimizing an influence on healthy cells, the ADC technology may have more excellent efficacy than that of a therapeutic antibody itself, and significantly decrease the risk of an adverse reaction as compared to existing anticancer drugs.
A basic structure of the antibody-drug conjugates as described above is composed of an “antibody-linker-low molecular weight drug (toxin)”. Here, the linker should allow the drug to exhibit a drug effect on target cancer cells while being easily separated by an antibody-drug dissociation phenomenon (for example, a result caused by hydrolysis by an enzyme) after the drug stably reaches to the target cells at the time of circulation and is introduced into the cells, in addition to playing a functional role of simply connecting the antibody and the drug. That is, since efficacy, systemic toxicity, and the like, of the antibody-drug conjugates depend on stability of the linker, the linker plays an important role in view of safety (Discovery Medicine 2010, 10 (53): 329-39).
The linkers of the antibody-drug conjugates developed up to now are roughly classified into a non-cleavable type and a cleavable type.
As a non-cleavable linker, thioether is mainly used, and a drug is not dissociated due to separation of the linker in cells, but the drug is dissociated in a form in which the drug includes the linker and a single amino acid derived from an antibody. In the case of a mainly used thiol-maleimide bonding method, a reaction is easily carried out at pH of about 6 to 7, but a reverse reaction may also be easily chemically carried out, such that there is a problem in stability.
As a cleavable linker, a linker separated by a chemical method or a linker hydrolyzed by an enzyme reaction is mainly used. As a linker having a chemical separation mechanism, a linker containing a disulfide bond is representative. In addition, hydrazone or oxime linkers are also used.
A disulfide linker, which uses a phenomenon that a drug is dissociated using a thiol exchange reaction, uses the fact that a concentration of thiol (particularly, glutathione) in cells is higher than that in blood. However, since various types of thiols (for example, albumin, and glutathione) are present in the blood, a drug may be separated during circulation. In the case of a hydrazone linker, it is known that the hydrazone linker is relatively stable in the blood, but is unstable in cells, endosomes, or lysosomes, in which acidity is high, such that the hydrazone linker is rapidly hydrolyzed (Bioconjugate Chem. 2008, 19, 759-765; Bioconjugate Chem., 2010, 21, 5-13).
In order to solve the problem as described above, a linker hydrolyzed by an enzyme reaction in cells has been developed, and a peptide linker (for example, valine-citrulline) and a β-glucuronide linker belong thereto. Valine-citrulline and β-glucuronide are not directly connected to a drug but are bound to self-immolative groups, such that the drug is dissociated by a mechanism such as 1,6-elimination or cyclization mechanism after hydrolysis by an enzyme reaction, thereby exhibiting efficacy (Clinical Cancer Res., 2005, 11, 843-852).
It was reported that a valine-citrulline peptide linker is selectively decomposed by lysosome protease such as cathepsin B, and increases stability in the blood as compared to the hydrazone linker, which is chemically decomposed, and thus, an anti-cancer effect is increased (Bioconjugate Chem. 2008, 19, 1960-1963; J. Org. Chem., 2002, 67, 1866-1872). However, the peptide linker has hydrophobicity, such that there are disadvantages such as aggregation of prepared antibody-drug conjugates.
The β-glucuronide linker, which is recognized to thereby be hydrolyzed by β-glucuronidase, has high hydrophilicity unlike the peptide linker, such that at the time of binding the β-glucuronide linker to a drug having high hydrophobicity, solubility of antibody-drug conjugates may be increased. Examples of binding antibodies to various drugs (for example, monomethylauristatin F, monomethylauristatin E, doxorubicinpropyloxazoline (DPO)) using the β-glucuronide linker to prepare antibody-drug conjugates have been reported (Conjugation Chem., 2006, 17, 831-840; US2012/0107332). According to the report, the antibody-drug conjugates prepared using the glucuronide linker are significantly stable in rat plasma, but stability thereof in mouse plasma was not reported.
Therefore, the present invention is to develop an effective linker comprising a self-immolative group, capable of being more stable in the plasma, being stable in the circulation, and allowing a drug to be easily released in cancer cells to exhibit a drug effect.